As energy prices increase due to the depletion of fossil fuels and concern over environmental pollution escalates, the demand for environmentally-friendly alternative energy sources is bound to play an increasing role in the future. Thus, research into techniques for generating various kinds of powers, such as nuclear energy, solar energy, wind energy, and tidal power, is underway, and power storage apparatuses for more efficient use of the generated energy are also drawing a lot of attention.
In particular, the demand for batteries as energy sources is rapidly increasing as mobile device technology continues to develop and the demand for mobile devices continues to increase. Accordingly, a lot of research on batteries capable of satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries thin enough to be applied to products, such as cellular phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which exhibit high energy density, discharge voltage, and output stability, is also very high.
In addition, secondary batteries may be classified based on the shape of a battery case of each of the secondary batteries into a cylindrical battery configured to have a structure in which an electrode assembly is mounted in a cylindrical metal container, a prismatic battery configured to have a structure in which an electrode assembly is mounted in a prismatic metal container, and a pouch-shaped battery configured to have a structure in which an electrode assembly is mounted in a pouch-shaped case made of a laminated aluminum sheet.
Particularly, in recent years, a lot of interest has been directed to a pouch-shaped battery configured to have a structure in which such a stacked or stacked/folded type electrode assembly is mounted in a pouch-shaped battery case made of a laminated aluminum sheet because of low manufacturing costs, light weight, easy modification of the shape thereof, etc. In addition, the use of such a pouch-shaped battery has gradually increased.
Furthermore, secondary batteries may be classified based on the structure of an electrode assembly, which has a structure in which a positive electrode and a negative electrode are stacked in a state in which a separator is interposed between the positive electrode and the negative electrode. For example, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which a long sheet type positive electrode and a long sheet type negative electrode are wound in a state in which a separator is disposed between the positive electrode and the negative electrode or a stacked type structure in which pluralities of positive electrodes and negative electrodes each having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the positive electrodes and the negative electrodes. In recent years, in order to solve problems caused by the jelly-roll type electrode assembly and the stacked type electrode assembly, there has been developed a stacked/folded type electrode assembly, which is a combination of the jelly roll type electrode assembly and the stacked type electrode assembly, having an improved structure in which predetermined numbers of positive electrodes and negative electrodes are sequentially stacked in a state in which separators are disposed respectively between the positive electrodes and the negative electrodes to constitute a bi-cell or a full cell, after which a plurality of bi-cells or full cells is sequentially folded while being placed on a separation film.
One of the principal research projects for secondary batteries is to improve the safety of the secondary batteries. For example, a secondary battery may explode due to high temperature and pressure in the secondary battery which may be caused by an abnormal state of the secondary battery, such as internal short-circuit of the secondary battery, overcharge of the secondary battery with higher than allowed current or voltage, exposure of the secondary battery to high temperature, or deformation of the secondary battery due to drop of the secondary battery or external impact applied to the secondary battery.
Specifically, if a battery cell is damaged by a metal member penetrating into the battery cell from outside the battery cell, the metal member directly contacts electrodes of an electrode assembly, with the result that an internal short-circuit may occur in the battery cell or the battery cell may catch fire, thereby greatly reducing the safety of the battery cell.
FIG. 1 is a typical view schematically showing an example in which a conventional battery cell is locally damaged by a metal member.
Referring to FIG. 1, a battery cell 100 is configured to have a structure in which an electrode assembly 150, including positive electrodes 111 and 112 and negative electrodes 121 and 122 in a state in which separators 131, 132, and 133 are interposed respectively between the positive electrodes 111 and 112 and the negative electrodes 121 and 122, is mounted in battery cases 141 and 142. A portion of the battery cell 100 is damaged by a metal member 160, which has penetrated into the battery cell 100 from outside the battery cell 100.
The metal member 160 has broken through the battery case 141 and then penetrated into the battery cell 100, with the result that the electrode assembly 150 has been locally damaged. In this case, the positive electrodes 111 and 112 and the negative electrodes 121 and 122, which constitute the electrode assembly 150, directly contact the metal member 160. As a result, an internal short-circuit occurs in the battery cell 100, with the result that the battery cell may catch fire or explode.
Therefore, there is a high necessity for technology that is capable of fundamentally solving the above problems.